As high-speed image forming apparatuses high in quality of image, there are known copying machines and laser beam printers adopting an electrophotographic process. With the advance of digital technology in recent years, the requirement for the higher quality of image of the electrophotographic image recording apparatus is rising in the POD market to the consumer's market such as offices and homes, and the requirement for an image subjected to screen processing such as printing, as well as an image characteristic like a silver salt photograph which is very high in gradation and has a wide color reproducing range and good graininess has further been rising.
This electrophotographic image recording apparatus is such that light is applied to an image bearing member by light, e.g. a laser beam or the like, and an image is recorded by the amount of light applied at that time, and can form all images from a binary image such as a character to an image such as a photograph including a halftone. When reproducing the medium density at this time, use is made of a pulse width modulating process (PWM process) and an image processing technique such as a dither method or a density pattern method, whereby various patterns can be formed on the image bearing member.
Charged toner particles are made to adhere to the obtained pattern on the image bearing member, and are further transferred onto and fixed on paper to thereby obtain a final output image. As the toner particles used at this time, toner particles of four colors, i.e., cyan, magenta, yellow and black, are popular, and various improvements have been applied thereto in order to improve various image characteristics such as graininess, gradation, density, chroma and gloss.
Also, with a view to further enhance the quality of image, in Japanese Patent Application Laid-open No. H5-35038, Japanese Patent Application Laid-open No. H8-171252, Japanese Patent Application Laid-open No. 2000-231279, Japanese Patent Application Laid-open No. 2000-305339, Japanese Patent Application Laid-open No. 2000-347476 and Japanese Patent Application Laid-open No. 2001-290319, there is disclosed a technique of using a plurality of toners differing in density level from one another to realize further improvements in graininess and gradation reproducibility in a high light to halftone area.
In hard copies such as printed matters, a screen angle is provided in a dither pattern used for the outputting of each color to thereby keep the uniformity of the color relative to the positional deviation of each color, and suppress the occurrence of moiré. Particularly, the occurrence of moiré is greatly affected by the combination of the screen angles of the respective colors, and as the combination widely spread in printing apparatuses, etc., mention may be made of yellow 0°, cyan (or magenta) 15°, black 45° and magenta (or cyan) 75°.
In an electrophotographic image recording apparatus provided with developing apparatuses capable of developing with two or more kinds of dark color and light color toners differing in density from each other, it is often practiced to use dither patterns of the same screen angle for the toner kinds of substantially the same hue, e.g. dark cyan and light cyan, and set the screen angles of cyan (light cyan), magenta (light magenta), yellow and black, as described above.
If dither patterns of the same screen angle are used for toner kinds of substantially the same hue, e.g. dark cyan and light cyan, when the positional deviation thereof has occurred in a surface, the unevenness of density, strictly, the unevenness of color will occur. When for example, a belt transfer member made of resin is used as an intermediate transferring apparatus, positional deviation of the order of several μm to 150 μm occurs in an image transporting direction.
When a screen of 200 lines has been formed, the screen structure is repeated at a cycle of the order of 120 μm, and when the above-mentioned positional deviation of the order of 100 μm to 150 μm has occurred, the phase of the screen structure is just reversed and thus, locations juxtaposed and locations overlapping each other appear repetitively, and great density unevenness occurs.
In such a case, it is also practised to effect direct transfer from the image bearing member to a final supporting member, or adopt a cylindrical intermediate transferring drum of relatively high rigidity as the intermediate transferring apparatus, thereby avoiding the above-noted evil, but this has not led to a drastic solution.
Also, to combine screens having five or more kinds of different angles to thereby form an image, the degree of freedom of the setting of the screen angles is low, and a Rosetta pattern occurring when screens of different angles are combined occurs at a low frequency and becomes noticeable. It is preferable to dispose toner kinds of the same hue and differing in density, e.g. cyan and light cyan, or magenta and light magenta, adjacent to each other, and uniformly dispose cyan, magenta, yellow and black used in a conventional process, as has heretofore been done.
Also, as dither patterns used for toner kinds of the same hue and differing in density, it is more preferable to use dither patterns approximate to each other in the angle with respect to the image transporting direction. In a popular electrophotographic image recording apparatus, the deviation of the printing position often occurs in the image transporting direction, and the screen angle most difficult to be affected by the deviation of the printing position is in the same direction as the image transporting direction, and this owes to the fact that the influence of the positional deviation is smaller in the order of the angles nearer to the image transporting direction.
It is also possible to dispose dither patterns used for the toner kinds of the same hue and differing in density adjacent to each other, and dispose dither patterns lower in density, e.g. light cyan and light magenta adjacent to each other. By adopting such a construction, the screen angles of cyan, magenta, yellow and black which are the basic hues of subtractive color mixture are ideally disposed and the screens of the toner species difficult to notice to each other, whereby it becomes possible to minimize the influence of the Rosetta pattern and positional deviation upon color unevenness.